Hydroxyl radicals (.OH) are key players in chemistry in surface waters, clouds, and aerosols. Additionally, .OH may contribute to the inflammation underlying adverse health outcomes associated with particulate matter exposure. Terephthalate is a particularly sensitive probe for hydroxyl radicals, with a detection limit as low as 2 nM. However, there is uncertainty in .OH quantification using this method, and potential for interferences from some transition metals. Terephthalate reacts with .OH to form a fluorescent product, 2-hydroxyterephthalic acid (hTA), with a moderate dependence on pH and temperature. However, there is disagreement in the literature on the yield of the fluorescent product (YhTA), which introduces a large uncertainty in the quantification of OH. Additionally, TA and similar organic probes are known to complex Cu(II) at high concentrations; thus, if this reaction is important at lower concentrations, Cu(II) could reduce apparent hTA formation, and reduce activity of Cu(II) in target samples. Using a pH 3.5 dark ferrous Fenton system to generate .OH radicals, we find that YhTA = 31.5 ± 7%. This is about double the recent literature value measured, but in excellent agreement with earlier measurements. Additionally, we find that interactions between Cu(II) and hTA are small enough to be ignored at Cu(II) concentrations below ∼50 µM.

Oxidative stress mediated by reactive oxygen species (ROS) is a hypothesized mechanism for particulate-matter related health effects. Fe(II) is a key player in ROS formation in surrogate lung fluid (SLF) containing antioxidants. Humic-like substances (HULIS) in particulate matter such as biomass burning aerosol chelate Fe(II), but the effect on ROS formation in the presence of lung antioxidants is not known. We use Suwanee River Fulvic Acid (SRFA) as a surrogate for HULIS and investigate its effect on OH formation from Fe(II). For the first time, a chemical kinetics model was developed to explain behavior of Fe(II) and SRFA in SLF. Model and experimental results are used to find best-fit rate coefficients for key reactions. Modeling results indicate SRFA enhances Fe-mediated reduction of O₂ to O₂^– and destruction of H₂O₂ to OH to 5.1 ± 1.5 and (4.3 ± 1.4) × 10³ M^–1 s^–1 respectively. Best-fit rates for Citrate–Fe(II) mediated O₂ to O₂^– and H2O2 to OH were 3.0 ± 0.7 and (4.2 ± 1.7) × 10³ M^–1 s^–1 respectively. The kinetics model agrees with both the experimental results and thermodynamic model calculations of chemical speciation for 0 and 5 μg/mL SRFA, but both models are less successful at predicting further enhancements to OH formation at higher SRFA Concentrations.